5 1 



internal tissues are placed in communication with the exterior, 

 by means of channels of communication. We have an example 

 of these in the ' lenticels ' found among the cork cells of the 

 outer layer of the bark, which are in reality the stomata of the 

 cork layer of the stem, and each frequently, if not always, 

 corresponds to a primitive stoma of the epidermis, whose place 

 the layers of cork have taken. Apart from the stomata and 

 lenticels, there is little transpiration of fluid ; for the leaves, as 

 we have seen, though thin, have the exterior layer of their epi- 

 dermal covering rendered impervious to water, so that exhala- 

 tion of watery vapour can only take place from the unthickened 

 cell walls of the cells composing the mesophyll, and bordering 

 the intercellular spaces with which the stomata communicate. 

 If we place a stem bearing leaves under a bell-jar at a sufficiently 

 high temperature, the glass will soon be covered with drops of 

 water, in consequence of the watery vapour, which is given off 

 by the plant, being condensed. 



Transpiration is not merely a process of evaporation, but is 

 something beyond that. Evaporation is dependent on certain 

 external influences, such as the amount of heat and the degree 

 of saturation of the air ; it will increase as the temperature of 

 the surrounding air rises, and the degree of saturation by mois- 

 ture consequently decreases. Transpiration is influenced by 

 the same circumstances ; its amount is of necessity the greater, 

 the higher the temperature and the drier the surrounding air ; 

 but it is also dependent on the nature of the plant. Evapora- 

 tion takes place in proportion to, and varies directly as, the 

 actual amount of surface exposed. The amount of transpiration 

 is dependent on the number of the stomata, and their size : 

 hence its amount is different on the two surfaces of a leaf; the 

 under surface, which has the greatest number of stomata, hav- 

 ing also the largest amount of transpiration. That transpira- 

 tion is not, however, absolutely dependent on the number of 

 the stomata, is shown by the Lime-tree {Tilia Enropaea) where 

 on the upper surface of the leaves the proportion of stomata in 

 a square millimetre is nil, and on the lower surface the propor- 

 tion is sixty, while transpiration takes place from the upper 

 surface in the proportion of twenty, and from the lower in the 



